Fix abort error handling

[Hitulya]

Related to #326

Added error checking for the underlying IDBRequest from the event target, fixing generic 'AbortError' throwing. Added separate 'abort' event handler that fires when a transaction is aborted without causing an error within the transaction or its underlying requests.

[mtoac]

Do we have a plan to merge this change? Has high probability to get AbortError on iOS

[masonicboom]

I think what's going on is this:

const tx = db.transaction(storeName, "readwrite");
// idb immediately sets up abort handler at https://github.com/jakearchibald/idb/blob/main/src/wrap-idb-value.ts#L91.

const store = tx.objectStore("s");
await store.put(val);

// User backgrounds the app.
// iOS aborts the transaction (https://bugs.webkit.org/show_bug.cgi?id=202705).
// tx.done abort handler fires (https://github.com/jakearchibald/idb/blob/main/src/wrap-idb-value.ts#L86).
// This becomes an unhandled promise rejection, because we haven't reached the next line yet.

await tx.done;

To reproduce, try something like this:

try {
  const tx = db.transaction(storeName, "readwrite");
  const store = tx.objectStore("s");
  await store.add({ id: 1 });
  await store.add({ id: 1 ); // Intentionally cause a constraint failure.
  await tx.done;
} catch (err) {
  console.error("Caught", err);
}

In addition to the console.error message, you should also see an unhandled promise rejection, because the tx.done promise rejected due to the constraint failure, and this happened prior to await tx.done.